1. Field of the Invention
The present invention relates to a road surface condition estimating apparatus for estimating the condition of a road surface on which a vehicle is running, and more specifically to a road surface condition estimating apparatus which is provided with a correlation among a plurality of wheel motion amounts expressing a road surface .mu. characteristic for each road surface condition, and which estimates and outputs the road surface condition on the basis of the correlation.
The present invention also relates to a variation reduction processing apparatus and more specifically to a variation reduction processing apparatus which can be applied to a road surface condition estimating apparatus, an ABS (anti-lock braking system) apparatus, a driving force controlling apparatus, and the like, and to a road surface condition estimating apparatus to which the variation reduction processing apparatus is applied.
2. Description of the Related Art
Conventionally, an anti-lock brake control device has been proposed wherein, immediately prior to a vehicle's wheel shifting to a locked up state when the coefficient of friction .mu. between the wheel and the road surface surpasses a peak value of .mu., (hereinafter referred to as "a peak .mu." or "a peak .mu. value") this device can prevent the vehicle's wheels from being locked up by reducing the braking torque acting on the wheel and by controlling the coefficient of friction .mu. to follow this peak value.
Meanwhile, when a vehicle is running at a certain speed and the brakes are applied, then the vehicle's wheels slip on the road surface. It is known that the coefficient of friction .mu. between the wheels and the road surface varies, as is shown in FIG. 21 in relation to the slip ratio .lambda. expressed in formula (1) below. EQU .lambda.=(V-V.sub.w)/V (1)
in which V is vehicle speed (conversion into an angular speed) and V.sub.w is a wheel speed, and accordingly (V-V.sub.w) corresponds to a slip speed .DELTA.v. PA1 memory means for storing a gradient of the coefficient of friction .mu. with respect to the slip speed or a physical amount relating to said gradient as one of the wheel motion amounts expressing said correlation; and PA1 road surface condition estimating means for estimating and outputting a road surface condition on the basis of comparison between a converted value of the wheel motion amount for each road surface condition obtained by respectively converting at least one of detected values of the plurality of wheel motion amounts in accordance with said stored correlation and a detected value of the same amount as said wheel motion amount. PA1 converting means for converting a detected value of at least one of the plurality of wheel motion amounts into a converted value of another of the plurality of wheel motion amounts for each of a plurality of road surface conditions on the basis of a correlation between the plurality of wheel motion amounts for each of a plurality of road surface conditions provided in said converting means; and PA1 road surface condition estimating means for estimating a road surface condition by comparing said converted value with a detected value of said another of the plurality of wheel motion amounts. PA1 said correlation includes a correlation defined for each vehicle speed and a correlation between two wheel motion amounts other than the vehicle speed, PA1 said converting means converts a detected value of any one of said two wheel motion amounts into a converted value of the other wheel motion amount for each of a plurality of slip speeds on the basis of said correlation between said two wheel motion amounts, PA1 said slip speed calculating means calculates a slip speed on the basis of said converted value and a detected value of said the other wheel motion amount, PA1 said vehicle speed calculating means calculates the vehicle speed on the basis of the slip speed calculated by said slip speed calculating means and the wheel speed detected by said detecting means, and PA1 said road surface condition estimating means estimates the road surface condition on the basis of the vehicle speed calculated by said vehicle speed calculating means and said correlation defined for each vehicle speed. PA1 said detecting means detects at least vehicle speed and a braking force as said plurality of wheel motion amounts, and wherein said road surface condition estimating apparatus further comprises: PA1 maximum braking force selecting means for selecting the maximum braking force corresponding to said estimated road surface condition and said detected vehicle speed on the basis of the maximum braking force provided for each of said plurality of road surface conditions and said plurality of vehicle speeds, the road surface condition estimated by said road surface condition estimating means and the vehicle speed detected by said detecting means, and PA1 subtracting means for calculating the difference between the maximum braking force selected by said maximum braking force selecting means and the braking force detected by said detecting means. PA1 second detecting means for detecting a vehicle speed of said vehicle; PA1 third detecting means for detecting a second physical amount which represents slipperiness of said wheel and which is determined in accordance with a road surface condition on which said wheel is running, said first physical amount, and said vehicle speed; PA1 calculating means for calculating said second physical amount for each of a plurality of predetermined road surface conditions on the basis of said first physical amount and said vehicle speed; PA1 comparing means for comparing a detected track of said second physical amount detected by said third detecting means with respect to said first physical amount with each of calculated tracks of said second physical amounts calculated for each of the plurality of road surface conditions by said calculating means with respect to said first physical amount; and PA1 estimating means for estimating a road surface condition corresponding to a track closest to said detected track among said calculated tracks as an actual road surface condition in which said wheel is running on the basis of the comparison results of said comparing means. PA1 said calculating means calculates the second physical amount for each of a plurality of road surface conditions at each time the first physical amount changes by a predetermined amount from the time braking is started on the wheel, and PA1 said comparing means compares the second physical amount detected at said predetermined time, with each of the second physical amounts calculated for each of the plurality of road surface conditions and at each time the first physical amount changes by a predetermined amount, at each time the first physical amount changes by a predetermined amount from the time braking is started on a wheel. PA1 second detecting means for detecting a second physical amount which represents slipperiness of said wheel and varies in accordance with a change in said first physical amount; and PA1 reduction processing means for reducing variations in the second physical amount in accordance with said change in the first physical amount. PA1 second detecting means for detecting a vehicle speed of said vehicle; PA1 third detecting means for detecting a second physical amount which represents slipperiness and which is determined in accordance with a road surface condition on which said wheel is running, said first physical amount, and said vehicle; PA1 reduction processing means for reducing variations in the second physical amount detected by said third detecting means in accordance with a change in said first physical amount; PA1 calculating means for calculating the second physical amount for each of a plurality of predetermined road surface conditions on the basis of the first physical amount and the vehicle speed; PA1 comparing means for comparing a detected track of the second physical amount in which variation is reduced by said reduction processing means with respect to the first physical amount with each of calculated tracks of the second physical amount calculated for each of the plurality of road surface conditions by said calculating means with respect to said first physical amount; and PA1 estimating means for estimating a road surface condition corresponding to a track closest to said detected track among said calculated tracks as an actual road surface condition on which said wheel is running on the basis of the comparison result of said comparing means.
As shown in FIG. 21, in this .mu.-.lambda. characteristic, the coefficient of friction .mu. reaches the peak value at a certain slip speed (the area A2 in FIG. 21).
In Japanese Patent Application Laid-Open (JP-A) No. 1-249559, there has been proposed the following anti-lock brake controlling apparatus. The anti-lock brake controlling apparatus computes the slip ratio from the approximate vehicle speed and the detected wheel speed using the formula (1). The anti-lock brake controlling apparatus controls the braking force in such a manner that the computed slip ratio substantially coincides with a reference slip ratio (a slip ratio giving the peak .mu.) which has been previously set at a fixed value. Accordingly, the coefficient of friction .mu. is continually controlled at the peak .mu. value.
However, the slip ratio at which the coefficient of friction .mu. reaches the peak .mu. value varies in accordance with the road surface condition (the type of road surface on which the vehicle is running and the condition thereof) . Accordingly, when performing follow-up control according to the fixed reference slip ratio as in the prior art described in the publication mentioned above, the stopping distance may become too long depending on the road surface, or else the brakes may be controlled so that the coefficient of friction .mu. goes over the peak .mu. value, whereby there is the concern that the wheels may lock.
In order to solve this drawback, it is necessary to estimate the road surface condition and change the reference slip ratio in accordance with the estimated road surface condition. However, in the conventional art, there is no technique for accurately estimating the road surface condition.